Zones are great and they are often the right solution. There are limits. If you have the choice of putting 100 zones on one 4 socket system or 25 zones on each of four 1 socket systems, I'd probably recommend going with the four systems. Hot locks and inefficient algorithms happen.
For instance, the directory name lookup cache can greatly impact the performance of file operations. This is what makes it so that when you open /a/b/c/d/e/blah, the OS has a good chance of knowing how to open "blah" directly without first searching /a, /a/b, /a/b/c, /a/b/c/d, and /a/b/c/d/e. I don't have performance numbers handy (left at $job - 1 and $job - 2), but the default size is not great for a system that handles hundreds of thousands of files. If that sounds like a lot of files, count how many files are read as part of a reasonably large build. Then imagine there are dozens of them running concurrently. Or imagine that you are hosting git repos or a bunch of static web content.
The obvious answer is to just increase the size of the DNLC. The problem is that there are other parts of the system that behave poorly with a large DNLC. For instance, whenever anyone tries to unmount a file system, dnlc_purge_vfsp() is called. This walks the cache, looking for entries that are associated with the file system. Who cares, right? We hardly ever unmount file systems. Well, if you use the automounter (by default Solaris uses it for at least /home/*), every few minutes it is trying to unmount all of the automounted file systems. The purge happens before EBUSY can be detected so hot cached entries may be purged while adding contention on dnlc-related locks. What's worse, the automounter doesn't reset its inactivity timer when it hits EBUSY, causing more frequent attempts to unmount than you would otherwise expect. There are other DNLC bottlenecks on a Solaris NFS server when a client removes a file.
And then as you get to larger systems with more NUMA effects, these type of operations become even more expensive.
Operating systems are hard. Making them scale to an infinite number of processes and processors is impossible. At a certain point, it becomes beneficial to use smaller hardware and/or add VMs into the mix.